As will be appreciated herein below, except as otherwise indicated, aluminium alloy designations and temper designations refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminum Association in 2008.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
For many years metal ingots, particularly aluminium ingots, have been produced by a semi-continuous casting process known as direct chill casting or electro-magnetic casting. In this procedure molten metal has been poured into the top of an open ended mould and a coolant has been applied directly to the solidifying surface of the metal as it emerges from the mould. Such a system is commonly used to produce large rectangular-section ingots for the production of rolled products, e.g. aluminium alloy sheet products. There is a large market for composite ingots consisting of two or more layers of different alloys. Such ingots are used to produce, after rolling, clad sheet for various applications such as brazing sheet, aircraft sheet, clad automotive sheet and other applications where it is desired that the properties of the surface be different from that of the core.
The conventional approach to such clad sheet has been to hot roll slabs of different alloys together to “pin” the two together for example by means of welding, then to continue rolling to produce the finished product, for example as disclosed in U.S. Pat. No. 2,800,709. This has a disadvantage in that the interface between the slabs is generally not metallurgical clean and bonding of the layers can be a problem.
Several alternative methods to improve on the bonding between the core ingot and the cladding are described in the literature.
Patent application US-2005/0011630-A1 describes what is also known in the art as the FUSION®-process (being a registered trademark of Novelis), and whereby two different alloys are cast in an open ended mould and by the use of special arranged dividers the first alloy pool contacts the second alloy pool at a point where the temperature of a self-supporting surface of the first alloy is between the solidus and liquidus temperature of the first alloy, and whereby the two alloy pools are joined as two layers and cooling the joined alloy layers to form a composite ingot.
In U.S. Pat. No. 7,250,221 a batch method is described of producing a clad metal ingot suitable for rolling to form a clad metal sheet, and wherein the upper rolling face of a solid core ingot is provided with multiple undercut cavities which are blocked when casting a cladding layer onto the upper rolling face. Once the cladding layer is solid, the cavities are unblocked and filled with a molten metal to form a metal lug therein attaching the cladding layer to the core ingot. This is reported to allow the cladding layer to contract without physical constraint during solidification and cooling, thereby avoiding the generation of internal tension and possible cracking. This approach overcomes at least partly layer separation during handling and rolling of the composite ingot.